It is known that iron-antimony metallic oxide catalysts containing an essential elements (I) iron, (II) antimony, (III) at least one element selected from the group consisting of vanadium, molybdenum and tungsten, and (IV) tellurium are useful for oxidation, ammoxidation or oxidative dehydrogenation of organic compounds.
Such catalysts are described in U.S. Pat. Nos. 3,668,147, 3,716,496, 3,988,359, 4,083,804 and 4,370,279, and Japanese Patent Publication Nos. 19766/72, 19767/72 and 39839/79, all of which were issued to the present inventors.
Such catalysts catalyze the oxidation of propylene into acrolein and acrylic acid, and the ammoxidation of propylene and acrylonitrile. They also catalyze the oxidation of isobutene and t-butanol into methacrolein and methacrylic acid and the ammoxidation of isobutene and t-butanol into methacrylonitrile.
Moreover, by the use of such catalysts, methanol is oxidized into formaldehyde and ammoxidized into hydrogen cyanide, and n-butene is oxidatively dehydrogenated into butadiene.
These iron-antimony metallic oxide catalysts have good activity and life (duration of selectivity), and exhibit outstanding catalytic performance. Nevertheless, they may become deactivated gradually when used over a long period of time, depending on the specific conditions of reaction. The deactivation may be accelerated by accidents or inappropriate reaction conditions.
There are a variety of causes for deactivation, and remedies for deactivation are being investigated from several aspects. The causes are not yet completely understood and the remedies proposed so far are limited to changing the reaction conditions, replacing the catalyst partially or entirely, and removing the deactivated catalyst from the reactor and regenerating it. Such remedies are time-consuming and costly because they interrupt the reaction. An example of these remedies is described in U.S. Pat. No. 4,208,303.
Any method of recovering the catalytic performance without interruption of the reaction would be very advantageous.
One such method is disclosed in U.S. Pat. No. 3,882,159. According to the disclosure, the deactivated molybdenum-containing catalyst for fluidized-bed reactors used for ammoxidation of hydrocarbons is regenerated by being contacted with fluidized-bed particles composed of a substantially inactive carrier and molybdenum oxide during the course of the reaction. The regeneration is accomplished by replenishing the catalyst with the molybdenum component, because it is stated that molybdenum is lost from the molybdenum-containing catalyst in the course of reaction and this decreases the catalytic activity.
This process, however, has certain disadvantages. The particles that replenish the deactivated catalyst with the molybdenum component are composed of a molybdenum component and an inert carrier. These particles release the molybdenum component, leaving only the inert carrier. The particles of the inert carrier have a low specific gravity, and mostly escape from the fluidized-bed reactor, but it is very difficult to remove them completely. The unremoved inert carrier particles (e.g., silica particles) accumulate in the fluidized-bed reactor and dilute the catalyst when the regeneration process is repeated, thus causing further problems in maintaining catalytic activity. For this reason, the regeneration process described can be used only for a limited period.
This U.S. patent is concerned only with regeneration of a molybdenum containing fluidized-bed catalyst, wherein molybdenum oxide supported on an inert carrier is used as the regenerating agent. The above-described U.S. patent does not disclose a process for regenerating an iron-antimony metallic oxide catalyst. Furthermore, because it teaches as essential the use of an inert carrier for the replenisher, the process has the disadvantage that unremoved inert carrier particles accumulate in the reactor, diluting the catalyst and reducing catalytic activity.
U.S. Pat. No. 3,236,782 discloses a process for regenerating metallic oxide catalysts, at least a part of which is chromium, vanadium, molybdenum or tungsten. According to the disclosure, the regeneration process is accomplished by contacting the catalyst with a vapor of the above-listed metal present in the catalyst. This regeneration process is intended to replenish the chromium, vanadium, molybdenum or tungsten component which has escaped in the course of reaction. This process, however, requires a complex operation to introduce into the reaction system fresh component in the vapor phase. Further, this U.S. patent does not disclose a process for the regeneration of an iron-antimony metallic oxide catalyst, and only discloses the process when used in a fixed-bed reactor.